Wouldn't it be "w dupie"?
No, that would mean that first you'd have to be inside and then kiss.
Ok, this is seen as direction rather than position and thus needs accusative. In some other languages that distinguish between position and direction it would have been otherwise.
Good that Ashibaal brought it up, really.
( ͡° ͜ʖ ͡°)
już czeka pociąg twój...